Ceramics have been known for many hundreds of years and have been used as coatings or as fabricated parts and are employed wherever their characteristics such as durability, nonporosity, electrical conductivity or nonconductivity, and heat protection are required. One of the more recent ceramic materials is a silicon-carbon-oxygen system, named as a black glass, which can find use in certain situations where extremely high temperatures are present.
Traditionally, the introduction of carbon in glasses was made by impregnating porous glass with a concentrated solution of an organic compound and subsequently firing in a reducing or neutral atmosphere. The carbon-containing product is generally regarded as a composite containing carbon and silica. Elmer and Meissner (Journal of the American Ceramic Society, 59, 206, 1976) of Corning Glass Works reported that the annealing point of reconstructed 96% silicon dioxide glasses is markedly increased by incorporating carbon in porous glass. Furfuryl alcohol was used as the pyrolyzable organic compound. They attributed the increase of about 100.degree. C. in annealing point to the effect of hydroxyl removal from the internal surface of the porous glass by hydroxyl reaction with carbon. The resistivities of samples with less than 2% carbon content approached that of the glass whereas the electrical resistivities of carbon-containing silica with carbon between 4.5-7% are in the range of 1-3 ohm-cm, thus producing electrically conductive glasses. The highest carbon content in the final glasses they could produce is 8.59%.
Smith & Crandall reported in U.S. Pat. No. 3,378,431 a method of making carbon-containing glass by hot-pressing to sintering temperature a mixture of colloidal silica and an organic compound known in the trade as "Carbowax" (polyethylene glycol) in an oxygen-free atmosphere. The black glass obtained from the mixture of 33% "Carbowax" and 67% silicon dioxide showed the presence of 1.2% by weight of carbon. A devitrification-resistant bonded mass of vitreous silicon dioxide and carbon physically inseparable and microscopically indistinguishable from silica was obtained. The black glass has a low thermal diffusivity and more resistance to crystallization than pure vitreous silica. Devitrification temperature increased by 150.degree. C. to 1250.degree. C. as compared with colloidal silica.
Carbon-modified silica glass has been used as a composite matrix by Larsen, Harada and Nakamum (Report No. AFWAS-TR-83-4134, December, 1983, Wright-Patterson AFB, Ohio). In producing fiber-reinforced composites, the processing sequence includes slurry impregnation of silicon carbide fiber in an aqueous slurry of a carbowax (polyethylene glycol) and a silicon-containing compound known in the trade as Cab-O-Sil (a silicon dioxide powder manufactured by Cabot), layout of prepregged fiber tows, and hot-pressing. The composites thus obtained exhibited high porosity and brittle fracture indicative of low toughness. They concluded that the silicon carbide/black glass fiber composite is a promising material, although the property goals were not achieved. There is suspicion that the silicon carbide fibers may have degraded.
More recently, formation of carbonaceous ceramics has been carried out through the use of the sol-gel process. January discloses in U.S. Pat. No. 4,472,510 the use of the sol gel process to form monolithic glasses containing carbon through pyrolysis of the gels of organosilsesquioxanes, metal oxides and metal alkoxides. Monomann in Great Britain Patent 1,359,576 disclosed the formation of silicon and quartz fibers using silsesquioxanes as precursors. Their gelling process used selected organosilicon compounds for the preparation of the ceramic precursor based on the following reaction: EQU .tbd.Si--OR+H.sub.2 O.fwdarw..tbd.Si--OH+ROH (1) EQU .tbd.Si--OH+HO--Si.tbd..fwdarw..tbd.Si--O--Si.tbd.+H.sub.2 O (2)
in which R represents an organic radical such as alkyl groups and aryl groups such as phenyl group.
The uniqueness of the sol-gel process is the ability to obtain homogeneous, purer glassy products by low temperature processes. Also, the use of a liquid sol as the starting material allows the preparation of intractable monoliths of complicated shapes utilizing a liquid path. The advantages of such a procedure over the powder consolidation techniques, such as sintering and hot isostatic pressing, are their formability of complicated shapes and low temperature operation. However, monolithic black glasses produced via hydrolysis and condensation of organoalkoxysilanes are not practical because of the requirement for very long drying periods and delicate gelling conditions. For example, January prepared a 0.66 cubic centimeter methyltrimethoxysilane gel monolith over a drying period of about three weeks, which, upon pyrolysis, yielded a carbon-containing black glass monolith of density 1.6 grams per milliliter.
The very slow drying rate is necessary for reducing cracks during the gelation period. These cracks form as a result of the non-uniform surface tensions created by the evaporation of the split-off water or alcohol molecules in the hydrolysis (1) and condensation (2) reactions.
Silsesquioxanes have also been produced by titanium catalyzed redistribution of methylhydridooligo- and-polysiloxanes by R. M. Laine et al., in Chem. Mater. 1990, 2, 464-472. A gaseous by-product (methyl silane, b.p. -57.degree. C.) is produced during the redistribution reaction.
In the instant invention, a hydrosilylation reaction was used for the gelation process in place of the hydrolysis-condensation route. The hydrosilylation involves addition of silane (Si--H) to vinyl silane (Si--CH.tbd.CH.sub.2) to form an ethylene or methylene linkage as illustrated in the following equations: EQU .tbd.Si--H+CH.sub.2 =CH--Si.tbd..fwdarw..tbd.SiCH.sub.2 CH.sub.2 Si.tbd.(3)
or ##STR2## The features of the hydrosilylation reaction are such that there is neither a small molecule reaction product nor a weight loss during gelation and that the carbons in the ethylene linkage are bonded to the silicon atoms. This gelation reaction completely eliminates the drying problem inherent in the hydrolysis of organoalkoxysilane process. We also found that cyclosiloxane gels cross-linked by hydrosilylation reaction produced upon pyrolysis to high temperature in a non-oxidizing atmosphere high carbon content, high yield and high density black glasses.
N. Harada and M. Tanaka in U.S. Pat. No. 3,957,717 described and claimed an organopolysiloxane gel prepared from cyclosiloxanes and H. Lamoreaux in U.S. Pat. Nos. 3,197,432 and 3,197,433 claimed the product gel from reacting cyclosiloxanes containing hydrogens and vinyl groups. The basic idea of reacting silyl hydrogen groups with silyl vinyl groups is found in U.S. Pat. Nos. 3,439,014 and 3,271,362.
Monomann in Great Britain Patent 1,359,576 disclosed the use of a phenyl group rather than a methyl group as R in order to increase the carbon content of their products. By choosing phenyl group as R, the carbon weight percent can be increased to as high as ca. 30%. However, we have shown in our simulation experiments that the carbon present started to oxidize at 550.degree. C. in flowing air and was completely removed before 1000.degree. C. Therefore, the carbon derived from pyrolysis of the phenyl group is free carbon susceptible to oxidation while our invention results in a carbon-containing material that is oxidation resistant up to about 1400.degree. C.
Okamura et al. reported in U.S. Pat. No. 4,618,591 a method of making silicon carbide-carbon composite molded product by using polycarbosilane as the precursor for a matrix material. The polycarbosilane on pyrolysis forms microcrystalline silicon carbide with inclusion of low oxygen percentage, as indicated by their X-ray diffraction patterns. In contradistinction to this work, this invention produces materials that have different composition ranges and that are overwhelmingly amorphous with a few small diffraction peaks different from silicon carbide.
The stability of a soluble polymer was studied by thermogravimetric analysis by A. Zhdanov et al. and reported in the Russian Journal Vysokomolekulyarnye Soedineniya, Series A, 16 (10), 2345-50 (1974). They precipitated the highly branched, soluble polymer from the reaction mixture as powders by adding alcohols into the reaction vessel before the gel point. Their polymer was different from a network gel produced from a sol-gel process in that it contained a large amount of unreacted Si--H and Si--CH.dbd.CH.sub.2 groups and was readily soluble in aromatic solvents. Also, the polymer powder did not melt when heated up to 500.degree. C. They heated the soluble polymers at 10.degree. C. per minute up to a maximum of 780.degree. C. in both Argon and air and reported the thermogravimetric results as to weight loss at various stages of heating and as to the total weight loss involved. No weight change was observed beyond 780.degree. C. when heated in Argon at a rate of 10.degree. C./min. with a final yield of 87%. The Russians did not characterize the resultant product of this analysis and appeared to have no interest in this product.
In contradistinction to this prior work, this invention is concerned with the product of pyrolysis of the gel polymers formed from cyclosiloxanes as well as with the process to produce such a product. The product of our invention is a hard, glassy material which we call a black glass having oxidation-resistant carbon and which is very useful when cast as a monolith, or one piece object.